Are the ratings still more or less what they were or have they dropped massively through the season? If there wasn't a serious drop, or unless Fox has another (cheap, perhaps not fiction if it's a new one) show it now thinks could perform better than Fringe in the "slot of doom". Typically it's a slot they would much prefer not to have to develop a new show for...developping new shows is expensive, and this slot isn't good at all to launch a new fiction show as its audience development will be typically very slow... but they need something on the air... it sounds like they put Fringe there to last until it's wrapped up. The costs of Fringe are controlled, the team is all set, and it's a show that generates some revenues from DVD sales (and they care enough to have released a economy-pack of seasons 1-2 before releasing the third) and that's already sold worwide, mostly to specialized channels that won't drop it (those channels are a market Fox has always been a good seller to). Unless the ratings have now dropped to the point the show is costing way too much money, and perhaps season 3 had such pathetic DVD sales that make them fear the show is now doomed... it sounds to me that it's a boat afloat, not going fast but better than building a new boat, especially for that slot.

My guess is that the rumours are based off pure speculation. There's always tons surrounding the J.J. shows and the rumour mongers always have a 50% chance of being right!

The odds are greater for Alcatraz, which was expected to be the new Lost, darling of both the audience and reviewers (that and prizes count more than we think. Madmen has a nearly confidential audience in the US. After the acclaim it suddenly got picked up by a massive number of broadcasters worlwide - and that is what keeps it alive, and around here at least and I suspect in many other markets, it keeps high in the TV DVD sales, enough so they eventually released a new edition that feature the French dub) - and even sparked the idea of a more "US mass audience" friendly "retro" show with Pan Am, which looks like a flop (the Québécois actress on the show did a major blunder by announcing on her twitter long ago that the show got cancelled and the network had added a few episodes so the writers could wrap it up... the network has denied all this, and perhaps it's true the decision isn't made as the actress now has a recurring role in another series (pilot stage, for now) and her contract has the option to make her join the main cast if the series is picked up, but only if Pan Am gets cancelled). Alcatraz looks like it won't be a massive success in foreign sales - it won't fetch the price for Lost certainly (they'll know soon about this, for non-anglophone nations), and they are probably looking at very average DVD sales as well. The temptation to try to replace it with a new show rather than support what looks set to destined to be a "lame duck" is probably very high. The big international marketplace for TV shows is in the spring. It may be coincidental, but it's often a few weeks later (not always, sometime they do it before) the US broadcasters announce the final decisions to renew or cancel their shows. I know for a fact that when a foreign broadcaster buys a season of a show, there is a clause to give the option for the buyer to renounce the purchase if the show gets cancelled (meaning if Alcatraz gets cancelled, it will be done knowing it means most of signed deals for season 1 for broadcast next fall would fall through). I suspect the spring marketplace can be the "final straw" for some network shows, those the networks hesitate to renew on their US performance alone.